This invention relates in general to the detection of corrosion or the like within non-ferrous metallic structures.
The non-destructive testing of metallic structures for purposes of detecting corrosion thereof, is generally well known in the art. The detection of corrosion of pipelines for example, by computer processing of measurement data obtained from the pipeline wall surface, is disclosed in Ellmann et al. U.S. Pat. No. 4,909,091. Eddy current and ultrasonic measurement techniques are relied on for corrosion detection according to the Ellmann et al. patent. These techniques however limit corrosion detection to relatively shallow depths, and are rendered unsuitable by most coatings on surfaces of the materials being examined. Such coatings include paint, radar absorbing, sound absorbing and marine fouling materials. Tolerable coating thickness is furthermore limited by the "lift-off" effect of eddy current techniques, while ultrasonic techniques will not even tolerate thin layers of a porous coating or interfaces that produce extraneous reflections.
The use of a magnetic field generator to examine metallic products in order to detect material defects therein, is disclosed in Taliaferro U.S. Pat. No. 4,866,383. According to such Taliaferro patent, magnetic properties introduced into the product being examined when subjected to a magnetic field from the generator, cause relative movement between the product and generator sensed by a load cell attached to the generator. The technique disclosed in the Taliaferro patent is accordingly designed to differentiate between contaminants and other such defects within the product being examined.
It is therefore an important object of the present invention to provide a system and method for detecting and quantifying corrosion produced defects within a non-ferrous metallic structure at subsurface locations of substantial depth therein, especially when covered by a thick non-conductive coating.
An additional object of the invention in accordance with the foregoing object, is to provide a non-destructive method of examining a non-ferrous material by use of a generated magnetic field in order to detect corrosion of such material substantially below its external surface.